In a digital modulation method that uses one carrier wave (hereinafter referred to as “carrier”), the symbol period generally becomes shorter as the transmission rate increases. Under a multipath environment, this may lead to difficulty in demodulating a signal. The multipath refers to an environment in which a radio wave transmitted from a transmitting station arrives at a receiving station via a plurality of paths, which occurs due to reflection on an obstacle and the like. The similar environment as the multipath environment also occurs in a communication system in which a plurality of transmitting stations (or relay stations) transmit radio waves that carry the same signal. In the description below, a “multipath environment” refers to both of the above environments.
OFDM (Orthogonal Frequency Division Multiplexing) system has been proposed as a transmission system to improve the reception performance under a multipath environment. In the OFDM system, data are transmitted employing a plurality of carriers that are orthogonal to each other on the frequency axis. For this reason, the symbol period of data transmitted using each of the carriers is longer according to the OFDM system, resulting in less degradation of reception performance even under a multipath environment with large delays. In addition, a different demodulation method can be selected for each of the carriers.
According to the OFDM system, the transmitting station performs modulation using IFFT (Inverse Fast Fourier Transform), and the receiving station performs demodulation using FFT (Fast Fourier Transform). Therefore, the OFDM system has high frequency efficiency, and its application to digital terrestrial broadcasts has been widely explored. In Japan, ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), a digital terrestrial broadcast standard, has adopted the OFDM.
FIG. 1 is a diagram illustrating the configuration of an embodiment of an OFDM receiver used in a digital broadcast system. In FIG. 1, an OFDM signal is received by a tuner 101 and converted into a digital signal by an A/D conversion unit 102. An orthogonal demodulation unit 103 generates an orthogonal signal from the digital signal obtained by the A/D conversion unit 102. The orthogonal signal is a complex digital data stream representing an I component and a Q component. An FFT unit 104 performs Fourier transform (that is, FFT) for each symbol of the orthogonal signal obtained by the orthogonal demodulation unit 103. A time domain signal is converted into a frequency domain signal by the FFT. The frequency signal contains a data signal and a scattered pilot (SP) signal that are transmitted using carriers having different frequencies from each other. A transmission path equalization unit 105 corrects phase rotation occurring in the transmission path. An error correction unit 106 performs error correction and recovers the transmitted data.
An IFFT unit 107 converts the SP signal contained in the frequency domain signal output from the FFT unit 104 into the time domain signal. A delay information extraction unit 108 generates a delay profile as delay information, on the basis of the time domain signal output from the IFFT unit 107. The delay profile represents the variation of the reception power on the time axis. The delay information extraction unit 108 also generates an FFT window control instruction to instruct the position of the FFT window (that is, the calculation range of the FFT) and provide it to the FFT unit 104. The FFT unit 104 performs FFT for each symbol in accordance with the FFT window control instruction.
According to the OFDM system, a guard interval is introduced in order to increase the reception performance under a multipath environment. Hereinafter, the guard interval is explained, referring to FIGS. 2A and 2B. In FIGS. 2A and 2B, it is assumed that under a multipath environment where a main wave (desired wave) and a delay wave (undesired wave) are present, FFT is performed for symbol n of a received OFDM signal.
The FFT calculation is performed by inputting, into the FFT unit 104, information within the FFT window that is set on the time axis. The width of the FFT window corresponds to one symbol time. At this time, without a guard interval inserted between symbols, when obtaining information in a symbol n of the main wave, not only information in the symbol n of the delay wave but also information in a symbol n−1 of the delay wave are obtained, as illustrated in FIG. 2A. In other words, the data of the symbol n are to be recovered on the basis of the information in the symbol n and the information in the symbol n−1. This causes inter-symbol interference, decreasing the reception quality.
Therefore, according to the OFDM system, a guard interval is interested between symbols, as illustrated in FIG. 2B. A guard interval i (i represents a number for identifying each symbol) is obtained by copying information at the end of a symbol i. In Mode 3 of ISDB-T, a guard interval corresponds to ⅛ symbol period.
When the FFT window is set at the symbol timing of the main wave as illustrated in FIG. 2B, information in the symbol n of the delay wave and information in guard interval n of the delay wave are obtained with the acquisition of information in the symbol n of the main wave. However, the information in the guard interval n has been obtained by copying a part of the information in the symbol n. Therefore, in this case, the FFT calculation is performed only for the information in the symbol n. As a result, inter-symbol interference does not occur, improving the reception quality.
However, depending on the configuration of the communication system, an OFDM receiver may receive a main wave and its preceding wave. For example, in an SFN (Signal Frequency Network), a plurality of transmitting stations (or relay stations) transmit the same signal simultaneously. Now, it is assumed that the transmission power of a first transmitting station located near the receiver is low, and the transmission power of a second transmitting station located far from the receiver is high. Then, the signal from the first transmitting station arrives at the receiver earlier than the signal from the second transmitting station, but the wave received from the first transmitting station may be weaker than the wave received from the second transmitting station. In this case, the wave received from the second transmitting station is the main wave and the wave received from the first transmitting station is the preceding wave. A preceding wave is sometimes called “preceding ghost”.
In the case in which the interference wave is a preceding wave, the FFT window control at the symbol timing of the main wave as illustrated in FIG. 3A leads to the occurrence of inter-symbol interference. In other words, when obtaining information in the symbol n of the main wave, not only information in the symbol n of the preceding wave but also information in a symbol n+1 of the preceding wave are obtained. For this reason, when the interference wave is a preceding wave, the FFT window is controlled at the symbol timing of the preceding wave, as illustrated in FIG. 3B. Under this window control, demodulated data is obtained from information in the target symbol only.
A reception apparatus including a filter circuit, an equalization circuit, and a decision circuit described below has been known as an art related to OFDM. The filter circuit performs band restriction, using a plurality of filter coefficients having wider bandwidths than the guard interval contained in a transmission signal transmitted through the transmission path, to a transmission path estimation signal that is used for estimating characteristics of the transmission path. The equalization circuit equalizes the transmission signal using the transmission path estimation signal with the band restriction. The decision circuit detects the signal quality of the transmission signal after the equalization, and decides the optimal filter coefficient in accordance with the detection result.
A reception apparatus that calculates the delay time between the earliest-arrival signal and a delay wave (main wave) and decides the start timing of the FFT window on the basis of the delay time has been known as another art relates to OFDM.
A receiving method in which the FFT window position is corrected by calculating, using a delay profile, the delay time between the current FFT window position and the window position that should be applied to a preceding ghost, has been know as another art related to OFDM.
These arts are described in, for example, Japanese Patent Application Publications No. 2006-311385, No. 2001-292125, No. 2004-304618, and No. 2004-96187.
With OFDM, quality degradation due to inter-symbol interference can be suppressed by disposing a guard interval as described above. However, in a certain communication environment, a multipath delay larger than a guard interval may occurs. That is, the time difference between the main wave and an interference wave may become larger than the guard interval.
The occurrence of multipath delay that is larger than the guard interval inevitably results in the occurrence of inter-symbol interference, as illustrated in FIG. 4A and FIG. 4B. When the interference wave is a delay wave, the FFT window is set to extract the symbol of the main wave, as illustrated in FIG. 4A. In this case, not only the signal component in a symbol n but also the signal component in a symbol n−1 is extracted from the delay wave. Then, the signal component in the symbol n−1 of the delay wave interferes with the signal component in the symbol n of the main wave. However, the reception power of the delay wave is smaller than the reception power of the main wave. Therefore, the influence from the inter-symbol interference is, basically, not significant.
Meanwhile, when a preceding wave is present, the FFT window is set to extract the symbol of the preceding wave, as illustrated in FIG. 4B. In this case, the main wave acts as an interference component. Then, not only the signal component in the symbol n but also the signal component in the symbol n−1 are extracted from the main wave. Then, the signal component in the symbol n−1 of the main wave interferes with the signal component in the symbol n of the preceding wave. At this time, the reception power of the main wave is larger than the reception power of the preceding wave. Therefore, the influence from the inter-symbol interference is, basically, significant.
FIGS. 5A-5C are diagrams illustrating bathtub curves in the conventional arts. In FIGS. 5A-5C, the horizontal axis represents multipath delay. The positive value represents the time difference between the main wave and a delay wave, and the negative value represents the time difference between the main wave and a preceding wave. The vertical axis represents the D/U ratio to obtain a predetermined reception quality. The D/U ratio is the ratio of the powers of a desired wave (main wave) and an undesired wave (interference wave). For example, “D/U=3 dB” indicates that the predetermined reception quality can be obtained when the reception power of the undesired wave is 3 dB lower than that of the desired wave. FIGS. 5A-5C represents the simulation results in the case in which the modulation method is QPSK and only one undesired wave is present. In addition, FIGS. 5A, 5B, 5C represent the characteristics with the guard interval being 252μ seconds, 126μ seconds, 63μ seconds, respectively.
The reception quality is dependent on the size of the guard interval. Specifically, a smaller guard interval results in a narrower acceptable range for multipath delay.
In addition, the reception quality deteriorates more easily when the interference wave is a preceding wave, compared to the case where the interference wave is a delay wave. For example, it is assumed that the guard interval is 126μ seconds. In this case, as illustrated in FIG. 5B, when the interference wave is a delay wave, the reception quality remains good as long as the time difference between the main wave and the delay wave is no more than 260μ seconds. However, when the interference wave is a preceding wave, the reception quality deteriorates as the time difference between the main wave and the delay wave exceeds 250μ seconds.
Then, it is assumed that the guard interval is 63μ seconds. In this case, as illustrated in FIG. 5C, when the interference wave is a delay wave, the reception quality remains generally good as long as the time difference between the main wave and the delay wave is no more than 250μ seconds. However, when the interference wave is a preceding wave, the reception quality start deteriorating when the time difference between the main wave and the preceding wave exceeds 100μ seconds, and the reception quality significantly deteriorates when the time difference exceeds 200μ seconds.
Thus, in the conventional arts, the reception quality deteriorates easily, when a preceding wave is present in a communication system that uses OFDM, with a large time difference between the main wave and the preceding wave. For this reason, there has been a need for improving the reception quality in the presence of a preceding wave in a communication system that uses OFDM.